Impressive work on March. Bach would be proud.
On to tremelo flatpicking. Remember to hold your pick at a slight downward angle.
Just skim the top of your strings with the pick. Don't dig too deep.
Practice the exercises on page 48-49, and Blue Monday, Shenandoah (1/8 note tremelo) and then go back to some of the earlier songs from the books and try them with a tremelo.
Remember to tremelo within the rhythm (8th notes or 16th notes).